MSOs (multi-service operators) provide several services to end users through a fiber optic network, with the final connection to the user through a coaxial connection. The services provided by the MSO typically include broadcast analog video and narrow cast digital services (also referred to as sub-carrier signals), such as data, VoIP, subscription, pay per view and video on demand (VOD) services. The services are generally allocated a portion of an optical channel, which typically has approximately 1 GHz bandwidth available. While the bandwidth of a channel is generally constrained by the optical network (the optical network transmitters, optical fiber, channel filter bandwidths and coaxial connection), the number of users connected to the optical network continues to increase, which requires increased demand for bandwidth for the desired services.
In recent years wavelength division multiplexed (WDM) optical transmission systems have been increasingly deployed in optical networks to meet the increased demand for bandwidth by providing more than one optical channel over the same optical fiber. The WDM techniques include coarse wavelength division multiplexed (CWDM) and dense wavelength division multiplexed (DWDM) systems. Whether a system is considered to be CWDM or DWDM simply depends upon the optical frequency spacing of the channels utilized in the system.
FIG. 1 shows a simplified block diagram of conventional WDM transmission arrangements. As illustrated in FIG. 1, data or other information-bearing signals S1, S2, S3 and S4 are respectively applied to the inputs of modulators 2101, 2102, 2103, and 2104. The modulators 2101, 2102, 2103, and 2104, in turn, drive lasers 2121, 2122, 2123, and 2124, respectively. The lasers 2121, 2122, 2123, and 2124 generate data modulated optical channels at wavelengths λ1, λ2, λ3 and λ4, respectively. A wavelength division multiplexer (WDM) 214 receives the optical channels and combines them to form a WDM optical signal that is then forwarded onto a single optical transmission path 240.
Narrowcast signals may be RF frequency multiplexed into broadcast channels. The narrowcast signals are typically digital signals and are normally much lower in amplitude than broadcast video signals. The arrangement of sending the same broadcast signal and different narrowcast signals over multiple wavelengths (WDM) is a means of providing more segmentation in an optical network. Typically, the lasers 2121, 2122, 2123, and 2124 each receive a different narrowcast signal. The wavelengths carrying the combined broadcast and individual narrow cast signals, λ1, λ2, λ3 and λ4, respectively, are optically multiplexed onto optical fiber 240.
When the modulators 2101, 2102, 2103, or 2104 RF carriers over drive the laser, (e.g. drive the laser to produce laser pulses having an amplitude greater than the network or laser tolerances), clipping events will happen. The clipping events significantly impact a quadrature amplitude modulation (QAM) signal. Usually clipping will increase the bit error rate (BER) of the QAM signal. Although most QAM receivers use forward error correction (FEC), if the BER is too high, even FEC cannot recover the signal. When that happens the information may be lost.
One approach to avoid clipping is to use a delay line circuit and high speed peak detector to reduce the clipping events. The sub-carrier multiplexed signal passes through an RF delay line before being applied to the laser. Before the sub-carrier multiplexed signal goes to the delay line, a high speed peak detector is used to detect the clipping peak. The laser current is increased relatively slowly to prevent the bias current from producing signals at frequencies within the transmission band. Because the delay time is relatively long, the physical size of the coaxial delay line and the RF loss of the delay line make it difficult to be implemented.
Another approach is disclosed in U.S. Pat. No. 6,549,316 to Henry A. Blauvelt, which describes an anti clipping circuit which does not require significant delay of the main RF signal. The circuit design was based on statistical measurements of measured clipping events. The design includes a diode peak detection circuit that preferably generates a laser bias control signal that is proportional to the frequency and intensity of the clipping events, which is AC coupled to the lasers DC bias circuit. The approach also assumes that the clipping events happen randomly.
Accordingly, an improved method and apparatus for reducing clipping distortion is needed for a laser transmitter for reducing the BER of QAM signal transmitting.